1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods and apparatus for testing semiconductor chips.
2. Description of the Related Art
Soft defect isolation techniques are critical to the development and manufacture of large-scale integrated circuits such as processors. Examples of such techniques include those methodologies which exploit induced voltage alterations, such as Thermally Induced Voltage Alteration (“TIVA”), Light Induced Voltage Alteration (“LIVA”) and Charge Induced Voltage Alteration (“CIVA”). The basic setup for these techniques uses a constant current source for biasing the device under test (“DUT”) and a detector to sense the change in the voltage demand due to the localized heating, light stimulation by the laser or charge stimulation by an e-beam.
Another soft defect testing variant uses a scanning laser microscope to scan over the entire surface of a die on a pixel-by-pixel basis. While the laser spot dwells on a given pixel, a test computer causes the die to execute a test pattern or script. The laser radiation causes a local perturbation to the circuit. If a soft defect coincides with the location of the laser spot, then the test pattern will register an anomaly and the pixel location on the die in question will be flagged for further fault analysis.
Testing time is a limitation associated with the pixel-by-pixel approach. The test pattern must be run at least once for each pixel. The number of pixels that must be scanned individually for a given die will depend on the die size and the field of view of the laser microscope. A typical field of view is 512.times.512 pixels at a 5 times magnification. Under such conditions, a 1 cm.times.1 cm die will require the field of view to be moved twenty-five times to fully cover the die. Even though current test patterns require perhaps a few microseconds to a few milliseconds to run, the time for each test pattern multiplied by the sheer number of pixels can lead to hours or even days of testing time to locate just one soft defect. Care must also be taken into account for the potential for the scanning technique to produce a discontinuity between the length of time that the incident radiation beam strikes a given pixel and the response time of the signal detection circuitry. If the illumination time per pixel is less than the response time of the signal detection circuitry, then the sensed signal will tend to smear into adjacent pixels and produce a characteristic tail pattern that can obscure the behavior of adjacent structures.
One conventional technique to address the issue of testing time involves test pattern construction. Long and complex test patterns have the benefit of more fully exercising the circuits of the die and thus revealing more types of soft defects, particularly those that present very subtle characteristics. However, as a time saver, conventional test patterns are deliberately written to be relatively short, which limits the number and complexity of critical timing paths or soft defects that may be analyzed.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.